This invention pertains to a method and system for applying a thermally-printed indicia to a sheeting in a direction normal to the direction of movement of the sheeting past a print head.
Signs are commonly used along roadways to display information to motor vehicle drivers. A highway sign typically includes a retroreflective sheeting that has characters placed thereon. The characters form information that is of interest to the  motor vehicle drivers, and the retroreflective sheeting allows the information to be vividly displayed by the sign at nighttime. A retroreflective sheeting has the ability to return a substantial portion of incident light in the direction from which the light originated. Light from motor vehicle headlamps is retroreflected by the signs, allowing the information to be read more easily by passing motorists.
Highway signs tend to be fairly large in size to accommodate large characters. The characters are applied to the signs, typically, by screen printing or by use of cut-out characters. In screen printing, a positive or negative image of the characters is first provided on the screen. This often is accomplished by exposing non-masked portions of a photosensitive screen to light and removing the un-sensitized, masked regions by scrubbing. Ink is then forced onto the retroreflective sheeting through the openings in the screen where the photosensitive material was removed. Screen printing is the method of choice for making the more common signs such as stop and yield signs.
When a custom sign is needed, the cut-out character method frequently is used. Cut-out characters are made by die cutting each character or by electronically cutting the characters from a stock material such as Scotchlite(trademark) electronic cuttable film. The cut-out characters typically are secured to the underlying retroreflective sheeting by use of an adhesive or rivets. Although the screen printing and cut-out character methods provide suitable ways of placing characters on highway signs, these methods tend to be time-consuming and somewhat cumbersome.
Thermal printing has become a popular and commercially successful technique for forming characters on a substrate. Also referred to as thermal transfer printing, non-impact printing, thermal graphic printing, and thermography, thermal printing is a process by which a colorant is transferred with the aid of heat from a carrier to a thermal print receptive substrate. Thermal printing is more rapid than screen printing or cut-out characters, and it is less cumbersome and relatively simple to carry out in practice.
While thermal printing provides a rapid, wieldy means for placing information on a sheeting, this printing method also has its drawbacks. A major drawback is that known thermal printing apparatuses and  are unable to handle large sheetings. The presently known apparatuses generally are unable to print on sheetings greater than 16 cm wide. If a sign larger than 16 cm wide is desired, separate sheets must be printed on and those sheets must be subsequently joined together to produce the whole sign. Thermal printing has been used to place information on a retroreflective sheeting, however, the information that has been printed has been limited in size to images such as bar codes; see, for example, U.S. Pat. No. 5,118,930 to Takada.
FIGS. 1 and 2 illustrate an example of a known thermal printing apparatus 10 having a print head arrangement 11 that comprises driven roller 14 and a thermal print head 16. A thermal print receptive sheeting 12 is shown disposed therebetween. Thermal print head 16 may comprise heatable resistive elements in a thermal heating system.
If roll sheeting stock is used as depicted in FIG. 2, sheeting 12 is held upon a sheeting supply reel 26 and is collected at a sheeting take-up reel 28. Dancer rollers 25 along with supply reel 26 comprise suitable tensioning means for sheeting 12. Ribbon 24 is held upon and tensioned by reel 22 and is collected on driven reel 23. Sheeting 12 is transported in the direction generally indicated by arrow y across roller 14 by sheeting transport means known in the art, for example, a friction drive mechanism using a stepper motor. Print head 16 remains stationary and makes contact with thermographic ribbon 24 and transfers colorant from ribbon 24 to sheeting 12 as the sheeting 12 moves past the print head 16. When transfer of colorant is completed or is not to be applied, print head 16 may be retractably disengaged from ribbon 24 in the direction generally indicated by arrow z. Currently available thermal print arrangements may be referred to as xe2x80x9cdown webxe2x80x9d systems, because indicia are applied down the length of the sheeting while the sheeting is in motion. The thermal print head 16 is rectangular in shape and typically has a dimension S of about 10 to 16 centimeters, but thermal print heads having a dimension S of up to about 46 centimeters also are known. Ribbon 24 has the same width S, shown in FIG. 1, as print head 16. Dimension S determines the maximum printing width of sheeting 12 that can be printed upon with print head arrangement 11 in a single pass.
Because dimension S is limited in size, thermal printing has not found great commercial success in providing images on large sheetings such as on retroreflective sheetings used in highway signs. When a sign larger than S is desired, separate sheets must be printed on and those sheets must be joined together in registration to produce the whole sign. Another disadvantage of known thermal printing systems is that the wide ribbon has a tendency to wrinkle, causing an uneven transfer of colorant and poor quality indicia. Further, known systems do not use the ribbon in a very efficient manner. Thermal printing in a region having a width less than S results in using only the portion of the ribbon corresponding to the width of the printed image. The unused portion of the ribbon becomes discarded with the used portion and therefor results in unnecessary waste.
The present invention provides a method and systems which overcome the aforementioned drawbacks. Briefly, the method of the invention comprises thermally transferring colorant from a movable ribbon onto the printable areas of a thermal print receptive sheeting by the steps of:
a) moving a thermal print receptive sheeting past a thermal print head, where the thermal print head is elongate and has a length L of at least one centimeter that extends substantially in a first direction; and
b) transferring the colorant from the movable ribbon to the thermal print receptive sheeting while the thermal print head is moved in a second direction substantially normal to the first direction.
In one aspect, the system of the invention comprises: a) an elongate thermal print head for transferring colorant, having a length L of at least one centimeter arranged substantially in a first, xe2x80x9cdown webxe2x80x9d, direction that is parallel to the direction of travel of the thermal print receptive sheeting; b) a transport for moving the thermal print receptive sheeting past the thermal print head in the first direction and operably positioning the sheeting to receive colorant; c) a mechanism that moves the print head in a second, xe2x80x9ccross webxe2x80x9d, direction substantially normal to the first direction when the print head is transferring colorant; and d) at least one control device for coordinating print head engagement, colorant transfer, sheeting transport, and print head linear motion. The system may further comprise a mechanism for disengaging the thermal print head when the thermal print head is moved in the second direction and when no colorant is being applied to a printable area of the sheeting.
In another aspect, the system of the invention comprises a modular and transportable thermal printing system for transferring colorant from a ribbon to the printable area of a thermal print receptive sheeting. The modular and transportable thermal printing system has a frame assembly, a plurality of reel assemblies, a thermal printing mechanism, a print head moving mechanism, and at lest  least one control device. The frame assembly includes horizontal rail members and vertical rail members assembled in an open frame structure. The frame members have walls defining apertures suitable for receiving, in modular fashion, system components as needed. The reel assemblies operably hold, position, and rewind the thermal print receptive sheeting. Each of the reel assemblies has a mechanism for mounting onto the frame assembly. The reel assemblies transport the sheeting in a first direction. The thermal printing mechanism comprises an elongate, disengageable thermal print head having a length L of at least one centimeter arranged substantially parallel to the first direction for transferring colorant. The print head moving mechanism moves the print head in a second direction substantially normal to the first direction when the print head is transferring colorant. The system control mechanism operably controls print head actuation, print head retraction, colorant transfer, sheeting transport, and print head linear motion.
The method and systems of the invention are advantageous in that sheetings having a width greater than S (FIG. 1) now can receive a thermally printed indicia. The thermal printing method and systems of the invention also overcome the problem of ribbon wrinkling because they are able to use a ribbon having a narrow width. This is accomplished by having the sheeting stationary while the ribbon moves over the sheeting as the indicia is being printed thereon. Further, the method and systems of the invention provide more efficient use of ribbon and place less stress on the ribbon when in use. Also, a single ribbon can be used to print on sheetings of vary  varying widths. No longer does the ribbon need to be changed to accommodate varying sheeting widths.
The above and other advantages of the invention are more fully shown and described in the drawings and detailed description of this invention, where like reference numerals are used to represent similar parts. It is to be understood, however, that the drawings and description are for the purposes of illustration only and should not be read in a manner that would unduly limit the scope of this invention.